I have been an early user of QuantConnect since 2014 and the platform along with the team consistently managed to exceed my expectations. In particular, with the heavy lifting they did with handling Terabytes of minute-resolution options data (as well as tick data for equities and futures) all easily available in the cloud. A fairly extensive test coverage of the open-sourced Lean engine brings confidence in things working as expected. While there is no such thing as perfectly realistic backtesting, QuantConnect comes close enough with their reality modeling efforts. The Algorithm Framework also facilitates modular thinking about the building blocks of an investment/trading strategy. Overall - in my opinion, the best starting point available for serious and thoughtful diving into quantitative finance.